New Member Few problems with my .50 Caliber +

Hello everyone, new member here. I've had a Studio .22 + combo for about 10 years, really love it. I picked up a .50 Caliber + on ebay a few months ago for a pretty cheap price, I just wanted to try a different Mesa amp to see what it was like. This one has been modded so the reverb knob adjusts the lead gain, and the direct out knob on the back controls the reverb. When I turn the reverb up, especially if I have anything running through the effects loop, I get a high pitched ringing that won't go away unless I turn the reverb all the way down. It is similar to feedback but doesn't go away if you turn the volume knob on the guitar down. Also the amp behaves somewhat strangely, such as when using the clean channel, turning up the treble knob seems to increase the gain. The reverb also sounds really bad, almost like short slapback echo, but maybe that is how it is supposed to sound on this amp, but nowhere near as nice as my Studio .22 +. I haven't done anything to the amp yet, was thinking about ordering a new set of tubes but I don't want to sink a lot of money into it. I really like the lead channel, but prefer the clean channel of my Studio .22 +. I'm considering selling it but want to get it in good working order before I do. I can handle replacing components and soldering on circuit boards, I just don't know how to diagnose the problem. Thanks for any help!

Josh

Replace your V4 preamp tube with a known good one. It is micro-phonic. There should be a tube map in the cabinet somewhere.

You can also tap the tube with the eraser tip of a pencil (while the amp is on). If it's micro-phonic, you will hear it ring through the amp. But, I don't believe that is necessary.

Hate to say RTFM, but, http://www.mesaboogie.com/manuals/user_manuals.html#gpm1_2

It has good instructions on how to access the preamp tubes.

Thanks for the info, I didn't even know you could get a manual for this amp so thanks for the link.

Just had a chance to read over the manual, it looks like the tube for the reverb requires a 12AT7 not a 12AX7, which I don't have. Do you know if I could test it with a 12AX7 without damage? I plan on ordering some new tubes anyway.

Thanks!

12AX7 will work fine in that position temporarily. Could probably just leave it in there permanently. 12ax7 is just a higher gain version of a 12at7.

reverb tube microphonic ! change it

Well I tried replacing the reverb tube with 3 different good 12ax7 tubes, problem is still there. I also tried swapping out all of the other preamp tubes with a good tube, problem still there. One thing I did notice is the problem almost 100% goes away when using the lead channel, but returns when switching to the clean channel. And the reverb level seems to be reduced quite a bit when the gain on the lead channel is high, but if you roll back the gain the reverb comes back. I'm wondering if maybe someone screwed up when they added the lead gain mod? It certainly sounds like something has gone microphonic, can a component other than a tube do that?

I would have to agree that the mods are likely responsible for the behavior you are seeing.

How can I either redo or undo the mod?

Should be able to.

If it was my amp I would start with the schematic for the preamp:

http://www.freeinfosociety.com/electronics/schematics/audio/pictures/mesaboogie50caliberpluspreamp.gif

And work from there. Google images for 50 caliber + for shots of the inside.

However, if you are not familiar with amplifier repair, you can kill yourself if you don't know what you are doing. I would take it to a professional.

When I click the link I just get

Forbidden

You do not have permission to access this document.

If the amp isn't plugged in is there still a danger?

Yes, great danger. If you are not aware of this fact now, I would strongly recommend you take it to a professional to get it repaired.

An amplifiers capacitors store several hundred volts of electricity for a long time, or until discharged.

If you just bought it, and can return it, I would just return it, and find something else. Sounds like it might be more trouble than it's worth.

Just Google mesa boogie 50 caliber + schematic. That's how I found it.